We have demonstrated that fixation and erasure of phase holograms in ferroelectric materials can be achieved by fast electrical control. High ionic mobility that allows the change of the electronic pattern into a stable ionic one is obtained by applying, during 0.1 sec, an external field slightly smaller than the coercive field to the crystal. Erasure is accomplished in 3 sec by applying a field that causes saturation of the polarization. Investigation of this process was carried out in BaTio3 crystals doped with Fe.
Bi12SiO20 and Bi12GeO20 present the best known photorefractive sensitivity for read-write volume holographic storage (S−1≃300 μJ/cm2) combined with high-quality image reconstruction. Recording processes by photocarrier diffusion (no applied field) and by photocarrier drift are identified. The high photosensitivity is attributed to photocarrier displacements comparable to or larger than fringe spacings. Saturation diffraction efficiency at light power densities larger than 600 μW/cm2 at λ=514.5 nm occurs from complete photocarrier trap filling.
Holographic measurements of optically induced refractive index changes (light damage) of Fe doped KNb03 are interpreted in terms of photoconductivity results. In "undoped" KNb03 the light damage is one order of magnitude smaller than in LiNbO3.The light induced index changes can be drasticaUy reduced or enhanced by applying a relatively small dc-field along the polar f c axis. The space-charge field due to the "photovoltaic effect" is then increased or decreased by photoconductivity.The spatial frequency dependence of the light-induced index changes indicates, that diffusion processes of charge carriers are important for the light damage in KNb03 if the fringe spacing A of the recorded hologram is smaller than 2 pm. For A = 0.35 pm this term is two orders of magnitude larger thanjthe "photovoltaic effect" and the photoconductivity term.The dispersion of the absorption-and photoinduced currentdensity measurements yield basic information on the energy levels of the photorefractive centers controlling the index damage.
Bi12SiO20 and Bi12GeO20 are photoconductive and electro-optic crystals in which hologram recording is performed using the photorefractive effect; their sensitivity and optical quality are comparable with the high-resolution photographic plate. Hologram formation and erasure are analyzed in terms of a simple two-level charge transfer model. The photocarrier’s displacement length, comparable with fringe spacing and the nearly unity quantum efficiency, allows the attainment of the ultimate photosensitivity. The trapping centers concentration being much smaller than the absorption centers concentration, the saturation recording time and diffraction efficiency are found limited only by the trap-filling mechanism.
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